Understanding nervous system development and function requires elucidation of the relationships between the separate components of neural circuits and their function in an intact system. Revealing these relationships in intact systems has proven to be extremely difficult with conventional methods. The advent of multiphoton confocal microscopy, however, provides the opportunity for in vivo optical imaging deep within neuronal tissue with minimal photodamage. This method can be used to target electrode recordings to specific cell types or to optically probe the activity of identified cell types. Optical monitoring of neuronal activity can be done using measurements of fluorescent changes from calcium sensitive dyes or genetically expressed sensors of presynaptic vesicular release of neurotransmitter. In the present proposal, we plan to use multiphoton confocal microscopy, coupled with genetically encoded optical reporters and other small fluorescence molecules, to study the connectivity and the role of neural activity in the normal adult nervous system and in the formation of synapses during development and in disease states. These studies are only possible with the use of a multiphoton confocal microscopy system [unreadable] [unreadable] [unreadable]